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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.spark.deploy.master
import akka.actor.ActorRef
import org.apache.zookeeper._
import org.apache.zookeeper.Watcher.Event.EventType
import org.apache.spark.{SparkConf, Logging}
import org.apache.spark.deploy.master.MasterMessages._
private[spark] class ZooKeeperLeaderElectionAgent(val masterActor: ActorRef,
masterUrl: String, conf: SparkConf)
extends LeaderElectionAgent with SparkZooKeeperWatcher with Logging {
val WORKING_DIR = conf.get("spark.deploy.zookeeper.dir", "/spark") + "/leader_election"
private val watcher = new ZooKeeperWatcher()
private val zk = new SparkZooKeeperSession(this, conf)
private var status = LeadershipStatus.NOT_LEADER
private var myLeaderFile: String = _
private var leaderUrl: String = _
override def preStart() {
logInfo("Starting ZooKeeper LeaderElection agent")
zk.connect()
}
override def zkSessionCreated() {
synchronized {
zk.mkdirRecursive(WORKING_DIR)
myLeaderFile =
zk.create(WORKING_DIR + "/master_", masterUrl.getBytes, CreateMode.EPHEMERAL_SEQUENTIAL)
self ! CheckLeader
}
}
override def preRestart(reason: scala.Throwable, message: scala.Option[scala.Any]) {
logError("LeaderElectionAgent failed, waiting " + zk.ZK_TIMEOUT_MILLIS + "...", reason)
Thread.sleep(zk.ZK_TIMEOUT_MILLIS)
super.preRestart(reason, message)
}
override def zkDown() {
logError("ZooKeeper down! LeaderElectionAgent shutting down Master.")
System.exit(1)
}
override def postStop() {
zk.close()
}
override def receive = {
case CheckLeader => checkLeader()
}
private class ZooKeeperWatcher extends Watcher {
def process(event: WatchedEvent) {
if (event.getType == EventType.NodeDeleted) {
logInfo("Leader file disappeared, a master is down!")
self ! CheckLeader
}
}
}
/** Uses ZK leader election. Navigates several ZK potholes along the way. */
def checkLeader() {
val masters = zk.getChildren(WORKING_DIR).toList
val leader = masters.sorted.head
val leaderFile = WORKING_DIR + "/" + leader
// Setup a watch for the current leader.
zk.exists(leaderFile, watcher)
try {
leaderUrl = new String(zk.getData(leaderFile))
} catch {
// A NoNodeException may be thrown if old leader died since the start of this method call.
// This is fine -- just check again, since we're guaranteed to see the new values.
case e: KeeperException.NoNodeException =>
logInfo("Leader disappeared while reading it -- finding next leader")
checkLeader()
return
}
// Synchronization used to ensure no interleaving between the creation of a new session and the
// checking of a leader, which could cause us to delete our real leader file erroneously.
synchronized {
val isLeader = myLeaderFile == leaderFile
if (!isLeader && leaderUrl == masterUrl) {
// We found a different master file pointing to this process.
// This can happen in the following two cases:
// (1) The master process was restarted on the same node.
// (2) The ZK server died between creating the file and returning the name of the file.
// For this case, we will end up creating a second file, and MUST explicitly delete the
// first one, since our ZK session is still open.
// Note that this deletion will cause a NodeDeleted event to be fired so we check again for
// leader changes.
assert(leaderFile < myLeaderFile)
logWarning("Cleaning up old ZK master election file that points to this master.")
zk.delete(leaderFile)
} else {
updateLeadershipStatus(isLeader)
}
}
}
def updateLeadershipStatus(isLeader: Boolean) {
if (isLeader && status == LeadershipStatus.NOT_LEADER) {
status = LeadershipStatus.LEADER
masterActor ! ElectedLeader
} else if (!isLeader && status == LeadershipStatus.LEADER) {
status = LeadershipStatus.NOT_LEADER
masterActor ! RevokedLeadership
}
}
private object LeadershipStatus extends Enumeration {
type LeadershipStatus = Value
val LEADER, NOT_LEADER = Value
}
}
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